1. Field of the Invention
The present invention relates to a liquid crystal display device achieving imaging with a high S/N ratio using an invisible light source in addition to a visible light source.
2. Description of the Related Art
A liquid crystal display device has an array substrate in which signal lines, scanning lines, thin film transistors (TFT) and the like are formed. The device also has a driving circuit for driving the signal lines and the scanning lines.
Recently, as the technology in integrated circuit has been developed, TFTs and a driving circuit can be formed in an array substrate by polysilicon process.
This makes a liquid crystal device be miniaturized. And the device is widely used in a portable device such as a mobile phone and a laptop computer.
There is a liquid crystal display device that performs imaging. In this device, photoelectric conversion elements are arranged in an array substrate. The elements can also be formed in an array substrate by polysilicon process. For example, the elements are used to detect an area of a liquid crystal panel that is touched by a finger, and the like.
Such device is disclosed in Japanese Unexamined Patent Application Laid-open Nos. 2001-292276 and 2001-339640.
To obtain an image of a target, a capacitor is charged firstly. Then, a photoelectric conversion element discharges the capacitor based on amount of light that is reflected by the target to be received by the element. After a certain period of time, the device detects voltage of the capacitor and converts the voltage into an image.
By using the image of the target, a finger, for example, the device detects a position or motion of the finger. The device detects change in shape or darkness of a portion in the image corresponding to a portion of the liquid crystal panel which is touched by the finger, and the like. The portion in the image is hereinafter referred to as a touched-portion. On the contrary, the portion in the image corresponding to the portion that is not touched is hereinafter referred to as an untouched-portion.
By using result of detection, the device judges on whether the finger, and the like touches the panel or not. Such operation is hereinafter referred to as touch-detection.
When an illuminance of ambient light is high, the finger blocks the ambient light. An image in which the touched-portion is very dark while the untouched-portion is very bright is obtained. That is, an image with a high signal-to-noise (S/N) ratio is obtained.
In case that the ambient light is dark, the device receives a light that passes through the liquid crystal panel and is reflected by the finger. An image in which the touched-portion is very bright while the untouched-portion is very dark is obtained. That is, an image with a high S/N ratio is obtained.
In case that the ambient light is dark, in order to obtain an image with a high S/N ratio, it is necessary to increase a brightness of the liquid crystal panel. Increasing an intensity of light from a backlight can do it. Changing gradation values of an image to be displayed can also do it.
But in this case, it is preferable to decrease the brightness of the liquid crystal panel along with the intensity so as to reduce an amount of glare.
In case the brightness of the liquid crystal panel is decreased so as to reduce the amount of glare, or in case that an image which makes the brightness decreased is displayed, it becomes difficult to obtain an image with a high S/N ratio. And it becomes likely that misdetection in the touch-detection happens.
The present invention has been made in view of the foregoing points. An object of the present invention is to provide a liquid crystal display device capable of obtaining an image with a high S/N ratio even if the ambient light is dark and the brightness of the liquid crystal panel is decreased.